1. Field of the Invention
The present invention relates to reflectarray antennas for signal transmission to or reception from a geographic area whereby the reflectarray shapes the beam over the defined area.
2. Background Art
Radio frequency communication signals are transmitted or received via antennas. For example, a satellite antenna in geosynchronous orbit is typically designed to cover a geographic area. Conventional parabolic reflectors have been physically reshaped to form beams which are collimated over specified geographical areas. Reflectarrays can also be designed to form beams collimated over specific geographical areas.
Parabolic reflectors, when fed by a single radio frequency feed at the focus, generate pencil shaped beams. Optical techniques such as geometrical ray tracing demonstrate that all ray paths from the focus to any point on the reflector to the far field (on a reference plane), are of equal length. Consequently, such reflectors form focused pencil beams for all frequencies at which the feed operates. The pattern bandwidth of parabolic reflectors is thus limited only by the modest bandwidth variations which occur due to changes in the electrical size (wavelengths) of the reflector. These bandwidth variations are inversely proportional to the frequency of the signal waves, for example frequency increases of ten percent will reduce the bandwidth by the same amount.
Shaped reflectors generally have small variations in ray path electrical lengths, and consequently, the associated pattern bandwidths are relatively good. However, the reflector shape is unique for each different coverage area and thus the mechanical design and manufacturing process is highly customized for each different application. The cost and design/manufacture cycle times associated with these reflectors are driven by their customized shapes. It is known that performance similar to that of shaped reflectors can be achieved in a flat antenna with reflectarrays. Typically, a reflectarray includes a flat surface upon which surface elements perturb the reflection phase of the waves directed upon the surface so that the reflected waves form a beam over the desired coverage area in much the same manner as they do in an equivalent shaped reflector design. Significant cost and cycle time reductions can be realized with flat reflectarrays wherein a common surface shape, i.e., flat, is employed. Customized beam shapes are synthesized by varying only the printed element pattern on the reflectarray surface.
However, flat reflectarrays are subject to two pattern bandwidth limitations. The first limitation is due to variations in ray path electrical lengths that are inherent to reflectarray systems. The second limitation arises from reflectarray element phase variations as a function of the frequency of the wave impinging upon the element. These elemental effects further degrade the reflectarray bandwidth. As a result, attempts to configure the shape of the beam reflected from a reflectarray to a beam shape, defining a coverage area, are subject to losses that substantially reduce pattern bandwidth and thus limit the utility of the antenna for use over a band of frequencies.